1. Field of the Invention
This invention generally relates to a headlamp unit and more particularly to a headlamp unit including both an incandescent lamp and a high intensity discharge lamp.
2. Description of the Prior Art
Incandescent lamps, including halogen lamps having a filament made of a tungsten wire, are widely used as a lighting source in automotive headlamp units. These lamps, however, exhibit low efficiency and short life because of the filament radiation employed thereby. These filaments are also relatively easily broken due to vibration, and it is bothersome if the frequency of changing lamps increases. Moreover, there is in demand a headlamp unit having a greater lumen output.
It is considered, therefore, to use high intensity discharge lamps, i.e., small metal halide discharge lamps, having high lumen efficiency and long life compared to incandescent lamps. However, it is well-known that it takes a long time, from about scores of seconds to several minutes, for such a high intensity discharge lamp to attain a stable lighting operation.
It is known to use high intensity reflector lamps using an arc lamp, as shown in U.S. Pat. No. 4,345,178, on commercial aircraft for night landings. It is easy to apply a lamp only for general lighting use.
However, it is not known to apply such high intensity discharge lamps directly for use for motor vehicles, especially for automobiles, because it cannot obtain a sufficient lumen output for safe driving immediately after lighting such a lamp. Instead, it is necessary to wait more than scores of seconds for safe driving after lighting when only using such lamps.
Moreover, as is known, it is necessary that an automotive headlight provide two kinds of beams, i.e., a high beam for lighting long distance (for use in open country) and a low beam for lighting short distance (for use when passing vehicles traveling in the opposite direction). It is, therefore, necessary to be able to change freely between these two beams as occasion calls.